Prohibit using the same element multiple times in the #ast directive

3 files changed, 45 insertions, 4 deletions

diff --git a/grammar/grammar.go b/grammar/grammar.go
index c5b8a0c..b405429 100644
--- a/grammar/grammar.go
+++ b/grammar/grammar.go
@@ -752,10 +752,11 @@ func (b *GrammarBuilder) genProductionsAndActions(root *spec.RootNode, symTabAnd
 						continue LOOP_RHS
 					}
 					offsets[elem.Label.Name] = i
-				} else {
-					if elem.ID != "" {
-						offsets[elem.ID] = i
-					}
+				}
+				// A symbol having a label can be specified by both the label and the symbol name.
+				// So record the symbol's position, whether or not it has a label.
+				if elem.ID != "" {
+					offsets[elem.ID] = i
 				}
 			}
 
@@ -829,6 +830,7 @@ func (b *GrammarBuilder) genProductionsAndActions(root *spec.RootNode, symTabAnd
 						continue LOOP_RHS
 					}
 					astAct := make([]*astActionEntry, len(dir.Parameters))
+					consumedOffsets := map[int]struct{}{}
 					for i, param := range dir.Parameters {
 						if param.ID == "" {
 							b.errs = append(b.errs, &verr.SpecError{
@@ -850,6 +852,16 @@ func (b *GrammarBuilder) genProductionsAndActions(root *spec.RootNode, symTabAnd
 							})
 							continue LOOP_RHS
 						}
+						if _, consumed := consumedOffsets[offset]; consumed {
+							b.errs = append(b.errs, &verr.SpecError{
+								Cause:  semErrDuplicateElem,
+								Detail: param.ID,
+								Row:    param.Pos.Row,
+								Col:    param.Pos.Col,
+							})
+							continue LOOP_RHS
+						}
+						consumedOffsets[offset] = struct{}{}
 
 						if param.Expansion {
 							elem := alt.Elements[offset]
diff --git a/grammar/grammar_test.go b/grammar/grammar_test.go
index a90ee8b..2bb2358 100644
--- a/grammar/grammar_test.go
+++ b/grammar/grammar_test.go
@@ -772,6 +772,34 @@ bar
 			errs: []*SemanticError{semErrDirInvalidParam},
 		},
 		{
+			caption: "a symbol can appear in the `#ast` directive only once",
+			specSrc: `
+%name test
+
+s
+    : foo #ast foo foo
+    ;
+
+foo
+    : 'foo';
+`,
+			errs: []*SemanticError{semErrDuplicateElem},
+		},
+		{
+			caption: "a symbol can appear in the `#ast` directive only once, even if the symbol has a label",
+			specSrc: `
+%name test
+
+s
+    : foo@x #ast foo x
+    ;
+
+foo
+    : 'foo';
+`,
+			errs: []*SemanticError{semErrDuplicateElem},
+		},
+		{
 			caption: "the expansion operator cannot be applied to a terminal symbol",
 			specSrc: `
 %name test
diff --git a/grammar/semantic_error.go b/grammar/semantic_error.go
index c013cbc..a9543ef 100644
--- a/grammar/semantic_error.go
+++ b/grammar/semantic_error.go
@@ -34,6 +34,7 @@ var (
 	semErrDirInvalidName      = newSemanticError("invalid directive name")
 	semErrDirInvalidParam     = newSemanticError("invalid parameter")
 	semErrDuplicateDir        = newSemanticError("a directive must not be duplicated")
+	semErrDuplicateElem       = newSemanticError("duplicate element")
 	semErrInvalidProdDir      = newSemanticError("invalid production directive")
 	semErrInvalidAltDir       = newSemanticError("invalid alternative directive")
 )